Karla-Gard

In a parallel universe, the Karla-Gard were humanoid fungal beings with a collective consciousness that originally grew from "the rot of the [Time] Vortex" — specifically, the remains of Beasts of Time, Reapers and Vortisaurs — with the ability to "merge" with other life forms, taking over.

A group of Karla-Gard followed this universe's River Song's TARDIS across the Void into the Doctor's universe, intent on revenge after she "swiped their Spore Queen" in order to develop a gas which would kill them (creating a causal loop, as they were already pursuing her for this reason).

After an encounter with the Karla-Gard at the Tomb of River Song, River Song joined the other River in her TARDIS. The pair eventually defeated them by creating a temporal paradox by never inspiring Professor Dern as a child to go on her expedition to River's tomb. This paradox created the Bridge through which River and the Karla-Gard had originally passed through, at the Temple of Tears. (AUDIO: The Two Rivers)